Process for producing a thermal barrier in a multilayer system for protecting a metal part and part equipped with such a protective system

ABSTRACT

The object of the present invention is to produce a metal part equipped with a protection system, particularly for turbine blades for aircraft engines, having a thermal barrier that is improved in terms of thermal properties, adhesion to the part and resistance to oxidation/corrosion. In order to achieve this, the method according to the invention produces in a single step, from specific ceramics, coating layers using SPS technology. 
     According to one embodiment, a metal part is produced according to an SPS flash sintering method and comprises a superalloy substrate ( 22 ), a metal sub-layer ( 21 ), a TGO oxide layer ( 25 ) and the thermal barrier ( 23 ) formed by said method from at least two chemically and thermally compatible ceramic layers ( 2   a,    2   b ). 
     A first ceramic ( 2   a ), referred to as the inner ceramic, is designed to have a substantially higher expansion coefficient. The outer ceramic ( 2   b ) is designed to have at least lower thermal conductivity, and a sintering temperature and/or maximum operating temperature that is substantially higher. The thermal barrier ( 23 ) has a composition and porosity gradient ( 3 ) from the metal sub-layer ( 21 ) to the outer ceramic ( 2   b ).

TECHNICAL FIELD

The invention relates to a method for producing a thermal barrier in amultilayered system for protecting a metal part made of superalloy. Theinvention further relates to a metal part made of superalloy equippedwith such a protection system.

The field of the invention is the development of refractory materialscapable of constituting thermo-mechanical parts, particularly HP (HighPressure) turbine parts, such as rotor blades or distributors.

The ongoing improvement to the performance of modern gas turbinesrequires the use of increasingly higher turbine input temperatures andthus the use of materials with even greater refractory properties.

PRIOR ART

For this reason, nickel (Ni) and aluminium (Al) based superalloys havebeen developed, such as equiaxed superalloys, then directionallysolidified superalloys and, finally, single-crystal superalloys.However, the development of these superalloys is currently inadequate torespond to the ever increasing demands of high-temperature parts interms of lifetime. Typically, the maximum operating temperature forsuperalloys is approximately 1,100° C., whereas the temperature of thecombustion intake gases or turbine exhaust gases can significantlyexceed 1,600° C.

In this context, thermal insulating coatings for these superalloys haveemerged, allowing the temperature of the metal in parts cooled byinternal convection to be reduced. These thermal insulating coatings,referred to as thermal barriers, or even TBs, are generally constitutedby an outer zirconium oxide (or zirconia) based ceramic layer stabilisedby yttrium oxide (or yttria), also referred to as yttriated zirconia,deposited on a metal bonding sub-layer. The sub-layer is designed toprovide adhesion to the ceramic layer whilst protecting the metal of thepart against oxidation and corrosion.

The metal sub-layer can be formed by a galvanic deposit of platinum,followed by vapour-phase aluminising. The insulating ceramic layer ofyttriated zirconia is then deposited onto this sub-layer either bythermal spraying (in which case the microstructure of the resultingdeposit is of the lamellar type) or by electron-beam vaporisation of thematerial (in which case the microstructure of the resulting deposit isof the columnar type).

In order to improve the performance of TB coatings in terms ofresistance to oxidation/corrosion at high temperature, metal sub-layercompositions have been developed, such as sub-layers of compositions ofthe Ni_((1-x))Pt_(x)Al (nickel-platinum-aluminium) type. Platinum isdeposited onto the part by electrolysis, aluminium is deposited bychemical vapour deposition (CVD) or by physical vapour deposition (PVD).

Other developments have focused on improving the ceramic layer,particularly on the formation of the yttriated zirconia layer using thesol-gel method or cold plasma treatment.

DESCRIPTION OF THE INVENTION

The TB coatings provided by these developments remain limited in termsof performance and lifetime, particularly with regard tooxidation/corrosion resistance. Furthermore, the reproducibility of themethods used is not guaranteed, particularly for producing the coatingwith an Ni_((1-x))Pt_(x)Al sub-layer. In addition, the methods that areimplemented require a significant number of delicate and longoperations.

The specific object of the invention is to overcome these disadvantagesby proposing a method for producing improved TB compositions with evengreater refractory properties and substantially better resistance tooxidation and corrosion.

In order to achieve this, the method according to the inventionproduces, from a stack of specific ceramic layers, each layer havingspecific properties and functions, which differ and are even theopposite from one layer to the next, layers of coating in a single stepby applying Field Assisted Sintering Technology (FAST), in this caseSpark Plasma Sintering (SPS) technology.

SPS technology simultaneously combines the application of a uniaxialpressure and DC pulses in a controlled environment (vacuum or specificgases). This technology is known in the powder metallurgy field as itallows, by compaction and sintering, metal parts or oxides to beproduced from powders. In particular, the use of SPS technology allowsparts of microstructures to be manufactured that are controlled in termsof grain size and porosity.

More specifically, the object of the present invention is a method forproducing a thermal barrier in a multilayered system for protecting ametal part made of superalloy. The method consists in producing athermal treatment by flash sintering protection materials in superposedlayers in an SPS machine enclosure. These layers comprise, on asuperalloy substrate, at least two layers of zirconium-based refractoryceramics, a first ceramic layer, referred to as the inner layer, that ischemically and thermally compatible with the substrate, and a finalceramic layer, referred to as the outer layer, that is disposed over theother layers. This outer layer has higher physicochemical resistanceproperties, in relation to external pollutants of the CMAS type, and/orthermal resistance properties than the inner layer.

The effect of the physicochemical resistance is, in particular, awetting coefficient between the pollutants and the outer layer that issufficient to prevent the spreading and the penetration of pollutantsmelted on the outer layer. Advantageously, the outer layer can containan element, particularly cerium or another element from the group ofrare-earths which, in the event of chemical interaction with pollutants,increases the melting temperature thereof.

Preferably, the materials are selected so that the expansioncoefficients are high enough to follow the expansion of the superalloythat remains the coldest.

Advantageously, an assembly of metal sheets forming a metal sub-layercan be disposed between the superalloy substrate and the ceramic layers.

Preferably, the inner layer can have a thermal expansion coefficientthat is substantially higher than that of the final ceramic layer, inparticular a thermal expansion coefficient that is between that of thesubstrate and that of the final ceramic layer, and the outer layer ofthe thermal barrier can have a natural sintering temperature, as well asa maximum operating temperature, that is substantially higher than thatof the inner layer.

More specifically, the physicochemical resistance properties of theouter layer relate to sintering, corrosion, erosion and/or aerodynamics,these properties being implemented by a selection of ceramics thatrespectively relate to a thermal conductivity, a porosity, a hardnessand/or a roughness that is suitable and is reinforced by the thermaltreatment of the SPS machine. The outer layer thus has, relative to theinner layer, at least one of the properties selected from among thefollowing: lower thermal expansion, greater hardness, lower thermalconductivity, substantially higher sintering temperature, lower openporosity and/or less roughness.

In particular, thermal barrier porosities of between 15% and 25%, withporosity of less than 15% for the outer layer, are preferred. Theroughness of the outer layer is preferably less than 10 micrometers.

In fact, the thermal barrier has, due to the formation of compositionand porosity gradients, a gradient of passage between the functionexerted by each of the ceramic layers taken individually in relation totheir relative characteristics: the inner layer favours anchoring on themetal sub-layer, particularly on an alumina layer that forms on thesurface of the metal sub-layer, by virtue of its expansion coefficientthat is consistent with the thermal properties of this sub-layer andthose of the alumina layer. The result is that the constraints betweenthe sub-layer, the alumina layer and the inner ceramic layer areaccommodated. The outer layer provides greater thermal protection inoperating conditions, particularly in turbines, by virtue of being morerefractory than the inner layer, as well as having a higher sinteringresistance and maximum operating temperature.

Other resistance properties (erosion, corrosion) and aerodynamicimprovement by smoothing the outer layer can also be implemented by theselection of the material for the outer layer or of additional suitablelayers. In particular, a harder material provides better resistance toerosion. A material with low open porosity provides better resistance tohigh-temperature corrosion (for example, to pollution of the CMAS(calcium-magnesium-aluminium-silicate oxides) type. A small-grainmaterial provides less roughness and thus improves the aerodynamicproperties.

According to specific embodiments, with the enclosure being equippedwith pressurisation means and electric means for passing pulsed current,the pressurisation and the passing of the pulsed current are carriedout, in a production step, according to a flash sintering cycle that istemperature-, pressure- and time-controlled, with a temperaturethreshold of between 1,000° C. and 1,600° C., preferably between 1,100°C. and 1,400° C., and a pressure threshold of between 15 Mpa and 150Mpa, preferably between 10 Mpa and 100 MPa, so that the thermal barrierhas a composition, porosity and function gradient for anchoring to themetal sub-layer, on the one hand, and for external protection and/orsmoothing (in other words: roughness), on the other hand.

The invention further relates to a metal part made of superalloy, whichis equipped with a protection system comprising a thermal barrier and isproduced according to the aforementioned sintering method. The metalpart thus comprises a substrate, constituted by a nickel-basedsuperalloy, a metal sub-layer having platinum-enriched beta-(Ni,Pt)Aland/or alpha-NiPtAl phases, an aluminium oxide layer formed by thermalgrowth or TGO (“Thermally Grown Oxide”) when the part is produced byflash sintering, and a thermal barrier formed by said method from atleast two chemically and thermo-mechanically compatible zirconium-basedceramic layers and having an outer face. A first ceramic layer, referredto as the inner layer, disposed as close as possible to the metalsub-layer, is chemically and thermally compatible with this sub-layer,and a final outer ceramic layer, disposed as close as possible to theouter face of the barrier, is designed to have higher physicochemicaland/or thermal resistance properties than the inner layer.

Preferably, the inner ceramic is designed to have an expansioncoefficient that is substantially higher than that of the outer ceramicdisposed as close as possible to the outer face of the barrier. Thisouter ceramic advantageously has thermal conductivity that issubstantially lower than that of the inner ceramic, and a naturalsintering temperature and/or a maximum operating temperature that issubstantially higher than that of the inner ceramic. The thermal barrierhas a composition and porosity gradient from the metal sub-layer to theouter face, and a functions gradient for anchoring to the metalsub-layer, on the one hand, and for protecting and/or smoothing theouter face, on the other hand.

In these conditions, the thermal barrier has, in addition to the thermalstability properties, low thermal conductivity, a thermal expansioncoefficient close to that of the substrate and good resistance tosintering, as well as resistance to corrosion by chemical inertia inrelation to calcium-, magnesium- and aluminium-silicate-oxides by virtueof its granular microstructure with isotropic porosity, resistance toerosion and good aerodynamic properties, as well as excellent adhesionon the TGO layer.

The metal parts that are more particularly, but not exclusively,targeted are turbine parts or gas turbine compressor parts, inparticularly dresser, distributor or combustion chamber blades.

According to specific modes:

-   -   the ceramics are selected from “YSZ” compounds of zirconium        (ZrO₂) partially stabilised by yttria (Y₂O₃), “GYSZ” compounds        of YSZ doped with gadolinium oxide (Gd₂O₃), “LZ” compounds of        lanthanum zirconate (La₂Zr₂O₇), and “LZC” compounds of partially        ceriated lanthanum zirconates;    -   the inner/outer ceramics are advantageously selected from the        following pairs: xYSZ/LZ with a percentage x by mass of yttria        greater than or equal to 7%, xYSZ/LZC and xYSZ/GYSZ, in        particular x=7 and x=8;    -   the LZC compounds are LZyC(1−y), with y=70%, y and 1−y        representing the additional percentages of zirconium and        partially ceriated zirconate cerium, and the compounds of doped        YSZ are tGvYSZ, with a percentage by mass of gadolinium oxide        equal to 2% and a percentage v by mass of YYSZ equal to 8%.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentupon reading the following description, which relates to one embodiment,and with reference to the appended drawings, in which:

FIG. 1 is a partial schematic cross-section of an SPS tooling comprisinga matrix and pistons, in which an example of the assembly of layers of asample of a metal part according to the invention has been introduced inorder to complete flash sintering;

FIG. 2 shows an example of diagrams of temperature and pressureadjustment cycles as a function of time for the flash sintering of theaforementioned assembly, and

FIG. 3 is a cross-section of a sample according to FIG. 1 after thecompletion of the flash sintering.

DETAILED DESCRIPTION

The terms “vertical” and “horizontal”, “upper” or “lower”, and theirderivatives, relate to the elements as shown in the figures, i.e. duringoperating mode.

FIG. 1 shows a partial vertical cross-section of an SPS tooling 1 placedin a vacuum during use. The tooling 1 comprises a hollow graphite matrix10 surrounding a cylindrical chamber 11. An assembly of layers 2 of asample of a metal part according to the invention has been introducedinto this chamber for flash sintering to be carried out. The layersshown here are circular for forming a pin, in order to highlight thestructures obtained after sintering. The invention extends to theproduction of any type of part having a geometric shape adapted to theiruse by using a chamber or mould with suitable geometry.

Electric contacts 12 a. 12 b and 14 a, 14 b are arranged in the chamber11, either side of the assembly 2. The contacts 12 a and 14 a and 12 band 14 b, respectively, are disposed either side of an electricalbarrier 13 a or 13 b, respectively, in this case constituted by analumina powder. These contacts are made of flexible graphite, forexample. Papyex®. This material prevents the pollution of the mould andfacilitates removal from the mould. The electrical barriers limit thepassage of the current through the assembly 2, which current mainlypasses through the mould 10.

The tooling further comprises graphite terminals, 16 a and 16 b, forelectric power-up, with vertical longitudinal axes. These terminals alsoact as pistons that compress the contacts 14 a and 14 b either side ofthe assembly 2 by exerting an external load (F arrows).

The assembly 2 is more specifically constituted by a stack of metalsheets, constituting a metal sub-layer 21 between a superalloy substrate22 and ceramic layers 2 a, 2 b, and a thermal barrier 23 deposited as aceramic powder onto the sheets constituting the sub-layer 21 so as toform a system 24 for protecting the substrate 22.

In the example, the substrate 22 is an “AM1” Ni-based superalloy andcomprises tantalum (Ta), chromium (Cr), cobalt (Co), tungsten (W),aluminium (Al), molybdenum (Mo) and titanium (Ti). On this substrate themetal sub-layer 21 is constituted by a succession of 5 μm thick platinumsheets and of 2 μm thick aluminium sheets. The thermal barrier 23 isconstituted by two ceramic layers 2 a and 2 b successively added to thesub-layer 21 in the form of powders.

In the example, the layer 2 a, referred to as the inner layer, isconstituted by an 8YSZ ceramic powder, i.e. zirconia stabilised at 8% bymass of yttria. The layer 2 b, referred to as the outer layer, isconstituted by a 2G8YSZ ceramic powder, i.e. zirconia (ZrO2) partiallystabilised with yttria (8% by mass) and doped with gadolinium oxide(GdO2) or Gd at 2% by mass.

In other examples, the layers 2 a and 2 b are constituted by ceramicpowders, respectively 7YSZ/LZ and 7YSZ/LZ7C3 (i.e. with 70% of zirconiaand, in a complementary manner, with 30% cerium).

During the flash sintering operation, the temperature “T” and pressure“P” adjustment cycles as a function of time “t” follow the diagrams ofFIG. 2. The temperature diagram D_(T) reaches a first threshold P1 of700° C. after a temperature increase T1, with a ramp of 100° C. perminute. The first threshold P1 lasts for approximately 10 minutes and isfollowed by a second temperature increase T2, with the same ramp, forapproximately 10 minutes.

This second increase is followed by a third increase T3 with a lowerpitch (50° C./min) and duration (approximately 5 minutes) so as to reachthe second threshold, or the main threshold, P2. This second thresholdP2 is located in the 1,100-1,200° C. interval and lasts forapproximately 15 minutes. Temperature-controlled cooling R1 is carriedout for approximately 30 minutes with a pitch of the order of 20° C. perminute in order to reach approximately 500° C. This cycle lasts forapproximately one hour. This first temperature drop is followed by asecond natural cooling stage in order to reach the ambient temperature.

The pressure diagram D_(P) shows an extremely fast pressure increase A1from the atmospheric pressure of 0.1 Mpa to reach 100 Mpa in theexample. A pressure threshold P3 is maintained and lasts for asignificant part of the operation, for example for 40 to 50 minutes. Thepressure drop A2 is operated over a very short time in order to returnto the atmospheric pressure.

A 3-dimensional sample of a metal part coated with a protection system,according to the invention, provided by flash sintering is shown in thecross-section of FIG. 3. It is composed of the superalloy substrate 22covered with a protection system comprising, in successive layers, themetal sub-layer 21, an alumina layer 25, referred to as the “TGO” layer,and the thermal barrier 23 composed of inner 2 a and outer 2 b ceramics,initially made of layers separated before the flash sintering operation.

The outer ceramic 2 b has relatively low thermal conductivity, between0.8 and 1.7 Wm⁻¹K⁻¹ before consolidation and less than 0.8 afterworking.

In addition, the maximum operating temperatures of the ceramics 2 b and2 a are equal to 1,200° C. and 1,600° C. and more, respectively.Furthermore, the outer ceramic 2 b does not exhibit natural sintering upto temperatures of 1,600° C. or more.

Moreover, the outer ceramic 2 b advantageously has an expansioncoefficient that is substantially higher than that of the inner ceramic2 a, which is 10.4·10⁻⁶K⁻¹. The difference between these expansioncoefficients governs the lifetime of the assembly, particularly theadhesion of the ceramic to the TGO oxide that formed during SPSsintering.

Furthermore, the granulometries of the initial powders of the twoceramics have been selected so that the inner layer is ultimately lessdense than the outer layer. The denser outer layer can then more easilystop the pollutants of the CMAS (calcium-magnesium-aluminium-silicateoxides) type that cannot penetrate said outer layer. The less denseinner layer thus more easily accommodates the deformations of thesubstrate and the sub-layers.

In addition, the thermal properties of the outer ceramic 2 b providegood resistance in operating conditions, particularly in turbines inwhich the gas temperatures can reach 1,600° C. or more.

FIG. 3 also shows the porosity gradient G1 of the thermal barrier 23,with an increase in the size of the pores 3 of the layers of the thermalbarrier 23 from the outer face 2 a to the TGO layer. A ceramiccomposition gradient G2 of the barrier 23 is also shown, with theinterpenetration of the initial layers of ceramics 2 a and 2 b in anintermediate zone of the barrier 23. These gradients result in aprogressive gradient of variation of the initial properties of the twoceramics between the TGO layer, where the properties are those of theinitial inner layer 2 a, and the outer face 2 e, where the propertiesare those of the initial layer 2 b. There follows a progressive gradientof the properties and thus of the functions of the thermal barrier 23,ranging from compatibility with the metal sub-layer to a thermalprotection function on the outer face 2 e.

The invention is not limited to the examples that have been describedand shown herein. It is, for example, possible to combine more than twoinitial layers of ceramics, for example, three or four layers ofchemically and thermo-mechanically compatible ceramics. Advantageously,these layers have properties and thermal functions that vary in the samedirection between the first inner layer closest to the metal sub-layerand the outer layer deposited over the other layers. The first innerlayer has thermo-mechanical properties compatible with those of themetal sub-layer, and the final outer layer has the most resistantthermal properties in terms of use in temperature conditions that areequal to or greater than approximately 1,600° C. It is also possible toadd a layer that is only designed to protect the assembly againstcorrosion from CMAS and/or to improve the aerodynamics by smoothing thethermal barrier.

The invention claimed is:
 1. A metal part made of superalloy, equippedwith a thermal barrier being formed by a flash sintering operation in anSPS machine enclosure, wherein the metal part comprises a substrate, anda thermal barrier comprising at least two ceramic layers consisting ofzirconium-based refractory ceramic, at least one layer is an innerceramic layer of zirconium-based refractory ceramic, and at least onelayer is an outer ceramic layer of zirconium-based refractory ceramichaving an outer face and being disposed over the inner ceramic layer,wherein the thermal barrier has a porosity gradient, with porosityincreasing from the outer ceramic layer to the inner ceramic layer withregard to the metal part, wherein (i) the outer ceramic layer has atleast one physicochemical resistance property tocalcium-magnesium-aluminum-silicate oxide pollutants which is higherthan that of the inner ceramic layer, or (ii) the outer ceramic layerhas a higher thermal resistance property than that of the inner ceramiclayer wherein the thermal barrier formed by the flash sinteringoperation is a monolayer having a gradient of properties from the metalpart to the outer face of the outer ceramic layer corresponding to thegradient of initial properties of the inner ceramic layer and the outerceramic layer, and wherein the inner ceramic layer has a higher thermalexpansion coefficient than the outer ceramic layer.
 2. The metal partaccording to claim 1, wherein the substrate comprises a nickel-basedsuperalloy.
 3. The metal part according to claim 1, wherein the metalpart comprises a metal sub-layer.
 4. The metal part according to claim3, wherein the metal sub layer has a platinum enriched beta-(Ni,Pt)Alphase, and an alpha-NiPtAl phase, or both, and a TGO oxide layer formedby thermal growth during flash sintering.
 5. The metal part according toclaim 3, wherein the inner ceramic layer is chemically and thermallycompatible with the sub-layer.
 6. The metal part according to claim 1,wherein the physicochemical resistance property of the outer ceramiclayer is at least one selected from the group consisting of sintering,corrosion, erosion and aerodynamics, wherein the property is implementedby a selection of ceramics that is at least one selected from the groupconsisting of thermal conductivity, porosity, hardness, and roughnessthat is reinforced by the flash sintering so that the outer ceramiclayer has, relative to the inner ceramic layer, at least one of theproperties selected from the group consisting of lower thermalexpansion, greater hardness, lower thermal conductivity, highersintering temperature, lower open porosity and less roughness.
 7. Themetal part according to claim 6, wherein the outer ceramic layer haslower thermal conductivity than that of the inner ceramic layer, and anatural sintering temperature, a maximum operating temperature, or both,that is substantially higher than that of the inner layer.
 8. The metalpart according to claim 6, wherein the thermal barrier has a compositionand porosity gradient from the metal sub-layer to the outer face, andfunctions for anchoring to the metal sub-layer, and for protecting, theouter face, smoothing the outer face, or both.
 9. The metal partaccording to claim 1, wherein the inner ceramic layer is selected fromthe group consisting of a YSZ compound of zirconia partially stabilizedwith yttria, a GYSZ compound of YSZ doped with gadolinium oxide, a LZcompound of lanthanum zirconate and a LZC compound of partially ceriatedlanthanum zirconates.
 10. The metal part according to claim 9, whereinthe inner and outer ceramic layers are selected from the groupconsisting of xYSZ/LZ, xYSZ/LZC and xYSZ/GYSZ, wherein x is a percentageby mass of yttria greater than or equal to 7% by mass.
 11. The metalpart according to claim 9, wherein the LZC compounds are LZyC(1-y),wherein y=70%, y and 1-y are additional percentages of zirconium andpartially ceriated zirconate cerium, and the doped YSZ compounds aretGvYSZ, wherein a percentage t by mass of gadolinium oxide is equal to2% and a percentage v by mass of YSZ is equal to 8% by mass.
 12. Themetal part according to claim 10, wherein x=7% by mass.
 13. The metalpart according to claim 10, wherein x=8% by mass.
 14. The metal partaccording to claim 1, wherein the porosity of the thermal barrier isbetween 15% and 25%.
 15. The metal part according to claim 1, whereinthe porosity of the outer ceramic layer is less than 15%.
 16. The metalpart according to claim 1, wherein the roughness of the outer ceramiclayer is less than 10 micrometers.
 17. A metal part made of superalloyand comprising an assembly of layers of material comprising: a substratecomprising a nickel-based superalloy, a metal sub-layer, and a thermalbarrier, said thermal barrier comprising at least one inner ceramiclayer consisting of zirconium-based refractory ceramic and at least oneouter ceramic layer consisting of zirconium-based refractory ceramichaving an outer face, said outer ceramic layer of zirconium-basedrefractory ceramic being disposed over the inner ceramic layer, wherein(i) the outer ceramic layer has at least one physicochemical resistanceproperty to calcium-magnesium-aluminum-silicate oxide pollutants whichis higher than that of the inner ceramic layer, (ii) the outer ceramiclayer has a higher thermal resistance property than that of the innerceramic layer, or (iii) both (i) and (ii), the thermal barrier being acontinuous structure having a porosity gradient with porosity increasingfrom the outer face of the outer ceramic layer to the oxide layer formedon the metal part, and wherein the inner ceramic layer has a higherthermal expansion coefficient than the outer ceramic layer.
 18. Themetal part according to claim 17, wherein the assembly has undergone aflash sintering operation.
 19. The metal part according to claim 18,wherein the metal part comprises an oxide layer formed thereon duringthe flash sintering operation.